How close is Earth to a runaway greenhouse?

Recent calculations suggest that the inner edge of the habitable zone around the Sun could be as far out as 0.99 astronomical units (AU)- much closer to the orbit of Earth than had been thought. This reopens the question of whether future increases in atmospheric CO2 might trigger a runaway or moist greenhouse. A runaway greenhouse implies complete ocean vaporization; a moist greenhouse implies that the stratosphere becomes wet, leading to ocean loss via hydrogen escape to space.

Previous studies (Kasting and Ackerman, 1986) had indicated that neither a moist nor a runaway greenhouse could be triggered by CO2 increases of any magnitude. Here, we revisit this question with a 1-D climate model that includes updated absorption coefficients for CO2 and H2O, along with an improved parameterization of tropospheric relative humidity. We find that a runaway greenhouse is still precluded. However, a moist greenhouse could conceivably be triggered by an 11-fold increase in atmospheric CO2, and humans could be subject to fatal heat stress for CO2 increases of (4-8)-fold. When this relative humidity parameterization is used in our habitable zone calculations, the inner edge moves inward to 0.97 AU. Both of these calculations remain overly pessimistic, as relative humidity may increase more slowly than assumed and cloud feedback is probably negative in this temperature regime.

Finally, we reexamine the lifetime of the biosphere against solar luminosity increases and show that older calculations suggesting ~0.5 Ga and 0.9 Ga as the lifetime for C3 and C4 photosynthesis are still approximately correct. Improvements in all of these estimates could be made with a properly formulated 3-D climate model that can self-consistently calculate relative humidity and cloud feedbacks.

Recent calculations suggest that the inner edge of the habitable zone around the Sun could be as far out as 0.99 astronomical units (AU)- much closer to the orbit of Earth than had been thought. This reopens the question of whether future increases in atmospheric CO2 might trigger a runaway or moist greenhouse. A runaway greenhouse implies complete ocean vaporization; a moist greenhouse implies that the stratosphere becomes wet, leading to ocean loss via hydrogen escape to space.